In general, the so-called “organic light emitting” phenomenon (organic electroluminescence) refers to a phenomenon in which electric energy is transformed into light energy by means of an organic substance. Particularly, when an organic film is disposed between an anode and a cathode and then electric potential is applied between both electrodes, holes and electrons are injected into the organic film from the anode and the cathode, respectively. When the holes and electrons injected as described above are recombined, excitons are formed. Further, when the exitons drop to a ground state, lights are emitted.
Generally, the organic film has a multi-layer structure including different materials in each layer so as to increase the efficiency and stability of the organic light emitting device using the same film. In order to obtain high light emitting efficiency, the amounts of the holes and electrons injected from both electrodes should be similar. However, there is a great difference between the movability of holes and that of electrons in one organic substance. Therefore, it is preferable that each of the hole transport layer, light emitting layer and an electron transport layer is formed individually to permit holes and electrons to be transferred efficiently to the light emitting layer, so that density of holes and that of electrons can be balanced, thereby increasing light emitting efficiency.
Light emitting materials used in the above-described organic light emitting device can be classified into fluorescent substances derived from excited singlet states and phosphorescent substances derived from excited triplet states. Further, they can be classified into blue, green and red light emitting materials depending on light colors, and yellow and orange light emitting materials needed for realizing more natural colors. The organic substance forming a light emitting layer in a light emitting device determines the color of the light emitted from the device. Additionally, substances emitting red, green or blue light are used appropriately to realize the full spectrum of color.
Meanwhile, host/dopant systems are generally used in order to increase color purity and light emitting efficiency by means of energy transition. The principle of a host/dopant system is as follows. When a host forming a light emitting layer is mixed with a small amount of a dopant having a smaller gap between both energy zones compared to the host substance, excitons generated from the host are transported to the dopant, resulting in highly efficient light emission. During the above process, wavelength of the light emitted from the host shifts to that of the dopant. Therefore, it is possible to obtain light with a desired wavelength depending on the kind of dopant.
Among such light emitting materials, red light emitting materials intrinsically have disadvantages in that energy efficiency is low, a so-called quenching effect occurs at high concentration due to intermolecular interactions through expanded pi (π)-electrons, and that color purity decreases due to a broad light emitting range. Such disadvantages of red light emitting materials were the most serious problems inhibiting practical use of full color organic light emitting devices. Due to the above problems, red light emitting materials would rather be used as a dopant together with a host than used alone. In the former case, it is possible to improve light emitting efficiency and color purity by using an energy transition from the host substance to the red light emitting material as a dopant, as described above.
However, host/dopant-based light emitting devices tend to show degradation of color purity because light emission caused by the host may occur or the dopant may be trapped at high voltage. Such problematic phenomena result from an insufficient energy transition from the host.
Hamada et al. suggested a solution for improving light emitting efficiency and color purity in [Applied Phys. Lett. 75, 1682 (1999)]. More particularly, they suggested an organic light emitting device having a light emitting layer comprising at least two dopants. However, it is very difficult to deposit a host substance along with two kinds of dopant substances at the same time in a practical process for manufacturing a light emitting device. Further, when either one kind of dopant or at least two kinds of dopants are used in a light emitting layer, wavelength of the light emitted from the light emitting layer shifts to the orange zone according to the increase of current density.
Accordingly, there is a continuous demand in the art for a light emitting material providing high light emitting efficiency and color purity.